pH-Defining Equilibrium between Water and Supercritical CO2. Influence on SFE of Organics and Metal Chelates

Abstract

A direct measurement of the pH of water in contact with supercritical CO2 was made by observing the spectra of a pH indicator with a UV-vis spectrophotometer. The pH was analyzed under pressures of 70-200 atm and temperatures of 25-70 °C. The measured pH varied from 2.80 to 2.95, and relative standard deviations of <1.5% were achieved. The effects of pH on the efficiency of supercritical fluid extraction of metals and ionizable organic species in water-containing systems are discussed.Supercritical fluid extraction (SFE) has become an attractive alternative to conventional solvent extraction for the recovery of organic compounds and metal chelates from solid and aqueous samples.1•2 Carbon dioxide is the gas of choice for SFE because of its moderate critical constants, inertness, low cost, and availability in pure form. In many environmental applications, water is often present in an SFE system, either as a part of the original sample or added deliberately. The presence of water has been shown to facilitate the extraction of metal ions from solid materials using the in situ chelation technique in supercritical C02.2 3 The role of water in the SFE of organics and metals is not well understood. Water in contact with carbon dioxide becomes acidic due to the formation and dissociation of carbonic acid: C02 + H20 -H2C03 -»H+ 4-HC03D etermining the pH of water in contact with supercritical C02 is essential to understanding SFE of both metal ions and ionizable organic species. For SFE of metal ions, complexing agents are used to neutralize the metal charge and to transport the metals into the CO2 phase. The degree of dissociation of the complexing agents is dependent on the pH of the solution. If the complexing agents are not ionized at the solution pH, complexation and

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